A substantial amount of interest has recently been centered on the development of ambient temperature, high energy density, electrochemical cells which are light in weight and capable of providing a higher voltage than conventional cells such as nickel-cadmium and lead-acid systems or alkaline cells having zinc anodes. The high energy density cell systems which are currently of interest typically involve the use of active metals (metals with reduction potentials which are more negative than that of hydrogen in the electromotive series of elements in an aqueous environment) as anodes in combination with nonaqueous electrolytes. As used herein, "nonaqueous" is intended to mean substantially free of water. Lithium has been of particular interest as an active metal for such high energy density cells since it is the most active of the metals in the electromotive series and has the ability in an electrochemical cell to provide the highest performance in watt-hours per kilogram of all known active metals.
In conventional electrochemical cells, cathode depolarizers are used in a form which will permit an external electrical circuit, such as a set of wires connecting the electrodes of a cell, while also effecting a physical separation of the cathode depolarizer from the anode. In such cells, the cathode depolarizer is generally an insoluble, finely divided solid which is either admixed with or used as a coating over an inert conducting material, such as a nickel or carbon rod, which serves as a current collector or cathode. The physical separation of the cathode depolarizer from the anode is necessary to prevent a direct chemical reaction between the anode material and the cathode depolarizer which would result in self-discharge of the cell.
Until recently, it was generally believed that a direct physical contact between the cathode depolarizer and the anode could not be permitted within an electrochemical cell. It has been discovered, however, that certain cathode depolarizers do not react chemically to any appreciable extent with active metal anodes at the interface between the anode and the cathode depolarizer. Accordingly, with materials of this type, it is possible to construct an electrochemical cell wherein an active metal anode is in direct contact with the cathode depolarizer. For example, U.S. Pat. No. 3,567,515 issued to Maricle et al. on Mar. 2, 1971, discloses the use of sulfur dioxide as a cathode depolarizer in such a cell.
U.S. Pat. No. 4,020,240, issued to Schlaikjer on Apr. 26, 1977, is directed to the use of clovoborate salts in an electrochemical cell which contains an active metal anode. Such salts contain an anion of the general formula (B.sub.m X.sub.n).sup.-k where m, n and k are integers, B is boron and X is preferably selected from the group consisting of F, Cl, Br and I. It is further disclosed that sulfur dioxide is a suitable electrolyte solvent and cathode depolarizer for such a cell and that Li.sub.2 B.sub.10 Cl.sub.10 is a suitable clovoborate salt. Similarly, U.S. Pat. No. 4,139,680, issued to Schlaikjer on Feb. 13, 1979, discloses that the above-mentioned clovoborate salts can be used as an electrolyte additive to prevent dendrite formation in alkali and alkaline earth metal nonaqueous secondary cells.
West German Offenlegungsschrift No. 2,140,146 discloses a nonaqueous electrochemical cell which contains an alkali metal anode, a strong oxidizing agent which undergoes reduction at the cathode and an electrolyte which is composed of a mixture of an alkali metal halide, an aluminum halide and sulfur dioxide. Chlorine, an interhalogen compound or an anhydrous salt such as copper (II) chloride is used as the oxidizing agent in this cell.
Copending U.S. Pat. Application Ser. No. 23,777, filed Mar. 9, 1987, now U.S. Pat. No. 4,752,541 discloses the use of aluminum chloride as an electrolyte component for lithium-sulfur dioxide electrochemical cells to improve discharge capacity and cycling characteristics. This copending application also discloses that a preferred electrolyte comprises a solution of aluminum chloride and at least one lithium salt in a mixture of liquid sulfur dioxide with at least one organic compound.
Rechargeable lithium-sulfur dioxide electrochemical cells are conventionally constructed using an electrode pack wherein alternating anode and cathode layers are separated from each other by an inert porous electrode separator. For example, the electrode pack can be prepared by sandwiching a porous electrode separator between a lithium foil anode and a sheet of flexible carbon as the cathode current collector and rolling the resulting sandwich structure into a roll. Electrode separators are conventionally fabricated from organic polymers such as polyethylene, polypropylene, polyvinyl chloride, nylon and copolymers of ethylene and tetrafluoroethylene.
We have found that conventional organic electrode separators undergo relatively rapid decomposition and failure when utilized in a rechargeable lithium-sulfur dioxide electrochemical cell which comprises aluminum chloride as an electrolyte component. This decomposition of the electrode separator is observed after a relatively small number of charge-discharge cycles and results in cell failure, apparently as a consequence of internal short circuits which are formed during recharge by lithium metal penetrating the decomposed separator material.